The present invention relates to telecommunication systems and more particularly to digital hierarchy systems (SDH). In such systems data is switched within an individual station to the appropriate output port. Also alternate routes are provided between stations by which data can be transmitted from an originating stations port to a receiver stations port so that failure of one route, e.g. due to accidental damage, will not prevent the data from reaching the receiver stations port via the other route.
The data is sent in the form of two packages or so-called virtual containers which essentially are made up of two sections, one being the actual data which is transmitted, i.e. the so-called payload, and the other being data which is concerned with the integrity of the data, referred to hereinafter as overhead or ancillary data.
One of the requirements for a system of the kind outlined above is that there must be some means for checking whether the package is correctly switched (or routed) at each individual station along the overall path of the data. Essentially this is achieved by use of the ancillary or overhead data area at each station in order to include an identity, and other monitoring information, to ensure correct switching.
In our U.K. patent application no. 9301575, filed Jan. 27, 1993, entitled Switch Protection Arrangement, now abandoned, we disclose an arrangement where the package is identified in terms of the channel number of the data, and the receiving stations card number. However there is a potential problem with such an arrangement in situations where the package travels by the alternative route referred to earlier. In the event of failure of the first route the package will acquire a new channel and card number. This means that the destination port of the station will not recognize the package because it will be looking for a package which has an identification which is characteristic of the first route. This problem is a direct result of the use of the alternate route protection identified earlier.
The present invention is concerned with overcoming this problem.
According to the present invention in a digital telecommunications system of the kind referred to, data packages carry identification data which is unique to the package within a station, and does not depend on channel or card numbers.
In designing systems of the kind referred to there is always the conflicting requirement of on the one hand making maximum use of the transmission capacity to transmit payload data and on the other hand ensuring the correct transmission of that payload data. In other words in each data package a balance has to be struck between the ratio of payload capacity and overhead or ancillary capacity.
Further design constraints are the internationally agreed standards and protocols which are necessary to ensure that the various national telecommunications systems can communicate with one another, e.g. the so-called CCITT standards and protocols.
Included in the ITU (formally CCITT) standards concerning the above mentioned overhead or ancillary data is one which calls for data whose purpose is to provide monitoring of the actual data, referred to as the V4 byte. This is only valid between stations, allowing it separate use within a station. In addition some data packages (or payloads) do not have such an overhead that may be used, in which case a part of the Section Overhead of the traffic is used in its place. For clarity only the V4 byte is described below, but the same principle applies to the Section Overhead.
According to one aspect of the present invention overhead or ancillary data characteristic of the identification of a data package is substituted for the so-called V4 byte to ensure the correct routing of the data package whilst at the same time not creating an increase in the overhead or ancillary data contained in the data package. In other words the inclusion of such identification data does not result in a requirement for increased bandwidth.
More specifically where an add-drop multiplexer (ADMUX) is employed to switch data from a data transmission channel into a so-called tributary channel the package identification data is substituted for the V4 byte as the package enters the ADMUX and is re-substituted as the package leaves the ADMUX.
The package identification data may have the characteristics referred to in the above-identified U.K. patent application number 9301575 or the characteristics referred to above in connection with the present invention.